⌘-K(or T) to open the channel switcher (I navigate channels solely with this shortcut and have my slack set to hide all channels without new messages)
⌘-F opens the search box. Can modify searches with from:@user and in:#channel for extra awesomeness
Up Arrow will edit you last message; perfect for when you hit Enter too early or mispell things
+:emoji: react to the last message. Have to be quick so nobody else replies before you react though or you'll react to the wrong message
/remind [someone usually myself] [to do something] [at this time in words] not really a shortcut but is great for not forgetting simple tasks
The channel switcher also supports fuzzy matching to some extent. If a channel is named foo-bar you can type fb to show it. It's doesn't work so well with multi-user-dm's though.
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